Multithreading Python: Thread Communication

Question

I'm using the threading module in Python and have read the tutorial here. My intended program flow is that I have 2 functions foo() and bar() and the following happens

1) foo() runs and collects some data stored in data then stops.

2) bar() then runs continuously using data.

3) After 2 minutes foo() runs again and updates data while bar() continues to run using the old version of data.

4) After the successful update bar() starts using the new version of data.

5) Loop back to (3)

Now I want to run these as separate threads and my issue lies in how does foo() tell bar() it has finished running and the new version of data is available?

I've read about Queue but couldn't see exactly how I would use it in this instance.

How does bar use the data? Does it keep using it over and over, or just once? If just once, then either the Queue or Pipe will work. — Hai Vu
– Hai Vu, Commented Mar 28, 2014 at 21:04

tdelaney · Accepted Answer · 2014-03-28 21:57:03Z

1

It depends on how bar consumes the data. If bar has some natural point where it can choose to use existing or new data, it can read a queue filled by foo.

def foo(bar_q):
    while True:
        data = get_some_data()
        # copy or deepcopy if foo will be updating data instead of fetching new
        bar_q.put(copy.copy(data))
        time.sleep(120)

def bar(bar_q):
    # wait for the first data
    data = bar_q.get()
    bar_q.task_done()
    while True:
        process_some_data(data)
        try:
            # get new data if available
            data = bar_q.get_nowait()
            bar_q.task_done()
        except Queue.Empty:
            pass

answered Mar 28, 2014 at 21:57

tdelaney

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1 Comment

rwolst Over a year ago

Thanks, it took me a while to get my head around the threading but its working!

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